Non-invasive system for breast cancer detection

ABSTRACT

A system for detecting an incipient tumor in living tissue such as that of a human breast in accordance with differences in relative dielectric characteristics. A generator produces a non-ionizing electromagnetic input wave of preselected frequency, usually exceeding three gigahertz, and that input wave is used to illuminate the living tissue, being effectively focused into a small, discrete volume within the tissue to develop a non-ionizing electromagnetic wave at that position. The illumination location is moved over a portion of the living tissue in a predetermined scanning pattern. Scattered signal returns collected from the living tissue are collected to develop a scattered return signal. The scattered return signal is employed to detect any anomaly, caused by differences in relative dielectric characteristics, that is indicative of the presence of a tumor in the scanned living tissue.

This application is a continuation-in-part of U.S. Ser. No. 08/269,691,filed Jul. 1, 1994 now abandoned.

BACKGROUND OF THE INVENTION

Breast cancer is one of the leading causes of death for women. About oneout of eight or nine women are expected to develop tumors of the breast,and about one out of sixteen to twenty are expected to die prematurelyfrom breast cancer.

Mammography or other X-ray methods are currently most used for detectionof breast cancers. However, every time a mammogram is taken, the patientincurs a small risk of having a breast tumor induced by the ionizingradiation properties of the X-rays used during the mammogram. Also, theprocess is costly and sometimes imprecise. Accordingly, the NationalCancer Institute has not recommended mammograms for women under fiftyyears of age, who are not as likely to develop breast cancers as areolder women. However, while only about twenty two percent of breastcancers occur in women under fifty, data suggests that breast cancer ismore aggressive in pre-menopausal women. Furthermore, women under fortyare getting the disease in increasing numbers--about eleven thousandannually now--and no one knows why.

Mammograms require interpretation by radiologists. One radiologist hassaid "I generally can spot cancers between five and ten millimeters indiameter. The prognosis is excellent then." However, about ten tofifteen percent of tumors of this size are not detected. One studyshowed major clinical disagreements for about one-third of the samemammograms that were interpreted by a group of radiologists. Further,many women find that undergoing a mammogram is a decidedly painfulexperience.

Thus, alternative methods to detect breast cancers are needed,especially those that do not entail added risks, that can detect tumorsas small as two millimeters in diameter, that are not unduly unpleasantto the patient, and that can be used for mass screening. A screeningsystem is needed because extensive studies have demonstrated that earlydetection of small breast tumors leads to the most effective treatment.While X-ray mammography can detect lesions of approximately five mm orlarger, the accuracy may range between 30% and 75%, depending on theskill of the diagnostic radiologist. Repeated X-ray examinations,however, are not encouraged because these may become carcinogenic. Theseconsiderations, in addition to cost considerations, have led physiciansto recommend that women wait until the age of fifty before havingroutine mammograms. One solution would be a non-ionizing, non-invasive,and low cost detection or screening method. It could greatly increasewithout hazard the number of patients examined and would identify thosepatients who need diagnostic X-ray examinations, where the added hazardsand costs could be justified. Thus, there is a need for a low-cost,non-invasive, screening method.

About one in eight women develop breast cancers and about one in sixteendie prematurely from this disease. Despite strong encouragement, lessthan half of the millions of women who should be are routinely screened.Some of the reasons are cost and discomfort experienced duringmammography. Other concerns are the additional risks associated withionizing radiation, especially for routine exams for women under fifty.However, while only twenty two percent of breast cancers occur in womenunder fifty, data suggest that breast cancer is more aggressive inpre-menopausal women. A screening procedure need only identify breastswith abnormalities. The precision and imaging requirements associatedwith diagnostic purposes and treatment monitoring, while desirable, neednot apply.

There are several generic detection methods: sonic, chemical, nuclearand non-ionizing electromagnetic. The sonic, chemical and nuclear (suchas MRI) techniques have been under study for some time and, while someinteresting approaches are being followed, none have been publicized asbeing available in the near future for low cost screening.

Non-ionizing electromagnetic methods have also been under investigation.Studies have considered the use of electromagnetic, non-ionizing methodsto detect or image portions of the human body. An excellent summary ofsuch activity is presented in a publication entitled "MedicalApplications of Microwave Imaging", edited by L. E. Larsen and J. H.Jacobi, IEEE Press 1986.* These activities include microwavethermography, radar techniques to image biological tissues, microwaveholography and tomography, video pulse radar, frequency modulation pulsecompression techniques for biological imaging, microwave imaging withdiffraction tomography, inverse scattering approaches, and medicalimaging using an electrical impedance. The publications in this bookcontain about five hundred citations, some of which are duplicates. Thetechnology cited not only includes electromagnetic disciplines, but alsonotes related studies in sonic imaging and seismic imaging. To updatethese data, the IEEE transactions on Medical Imaging, BiomedicalEngineering, Microwave Theory and Techniques and Antennas andPropagation have been reviewed. Also surveyed was the publicationMicrowave Power and Engineering. This update has indicated littlesignificant progress in the aforementioned electromagnetic techniquesthat would be important to detect

Many important reasons exist for this lack of progress. In the case ofmicrowave thermography, adequate depth of penetration, along with therequired resolution, may not be realized, except for large cancers. Inthe case of holography, reflections at the skin-air interface tend tomask the desired returns from breast tumors beneath the skin. Further,illuminating the entire volume of a breast either requires excessivepower (with possible biological hazards) or acceptance of poorsignal-to-noise ratios. In the case of through-the-body electromagnetictechniques, such as tomography, the attenuation characteristics of thebody are such that long wavelengths are usually used, with an attendantloss of resolution. Imaging by determining perturbations in bodyimpedance caused by the presence of tumors as sensed by multi-electrodearrays have been either inadequate in sensitivity or subject to falsealarms.

A millimeter wave FM radar weapons detection system developed and testedfor the FAA (DTFA03-87-C-00056) by the inventor employed a 94 GHz FMradar operating with a 300 Mhz bandwidth. A half-meter diameter antennawith a half inch spot size focused the radiated 94 GHz energy throughthe air onto a possible passenger boarding an aircraft. This systemsuccessfully detected both metallic and plastic weapons, with an overalldetection probability of 96.2%. The false alarm rate was 31.09%. It washoped, initially, that the system could be used to detect breastcancers, since there was some empirical evidence suggested that the 94GHz waves were penetrating the skin sufficiently that some portions ofthe shoulder blades could be resolved. However, subsequent research hasdisclosed that the air-skin interface would not only enlarge the spotsize, but would reflect a very substantial fraction of the impingingwaveform.

To mitigate the resolution problem, a much higher frequency is needed torealize a usable spot size. However, the use of higher frequenciesgreatly increases the path attenuation of the penetrating energy,thereby introducing major design difficulties. These findings largelynegated the use of this system for breast cancer detection.Nevertheless, the results of this FAA project suggested that at leastsome features of a millimeter wave weapons detection system, designedfrom existing data, could be revised and integrated into a successfulprototype system for detecting breast tumors.

SUMMARY OF THE INVENTION

The objective of this invention is to provide a system to propagatenon-ionizing electromagnetic waves having wavelengths not much greaterthan three times the circumference of the smallest tumor to be detected,preferably having wavelengths, in normal breast tissue, of the order ofthirty millimeters or less and preferably of the order of tenmillimeters. Propagation is effected without incurring intractable pathlosses, while at the same time being able to discern breast tumors ofthe order of three millimeters. The penetration is realized by: 1)avoiding interface reflections by employing media that have about thesame dielectric constant as the breast tissues; 2) choosing a frequency(and wavelength) that readily penetrates normal breast tissue; and 3)providing means to extract tumor-scattered power from the applied orimpinging power. The desired resolution is achieved by: 1) choosing afrequency such that its wavelength in breast tissue is comparable to theminimum size tumor to be detected; 2) using a wide aperture antenna thatfocuses the mmw energy at discrete points within the breast; and 3)relying on significant differences between the dielectric properties ofthe normal breast tissue and those of the breast tumors. Optimumoperation is usually achieved at frequencies in the range of three toninety gigahertz.

A principal feature of this invention includes means to introducemicrowave or millimeter wave energy into a breast with a minimum ofinterface reflections and loss of resolution (or increased spot size).This is done by means of dielectric materials in the illuminator thathave about the same relative dielectric constant as the breast tissueand by use of gels, liquids, slurries and/or solids that have a similarrelative dielectric constant around the breast to further suppressinterface gaps that could cause reflections and loss of resolution.

Another important feature is the selection of a band of operatingfrequencies wherein the attenuation of the propagating energy in anon-lactating breast is relatively small, preferably of the order of 1.5to 15 dB/cm, in combination with an antenna or illuminator aperture sizethat produces a spot size preferably in a range of about 2.5 to 12millimeters in normal, non-lactating breast tissue.

Another important feature comprises the use of a wide aperture scanningsystem. The construction of the scanner is such that the focus of theenergy introduced is scanned at different depths, at depth incrementscomparable to the depth of focus, to provide a quasi-three-dimensionalpicture of the backscattered returns. To overcome path anomalies thatmight cause ambiguous results, different scanning patterns can beemployed to average out such effects. Also, advantage can be taken ofother features inherent in electromagnetic propagation systems, such asthe use of different polarization effects, including circularpolarization, and enhancement of the backscatter cross-section whereinthe circumference of the tumor is equal to the wavelength. Also, use offorward and side scatter can be employed to help resolve ambiguities.

Yet another important feature employs techniques that aid in separatingthe desired scattered returns from a tumor from those originating eitherdirectly from the impinging waveform or from spurious reflections fromscatterers of no interest. This may be done by "passive methods", suchas employed in microwave circuits (magic tees or circulators) or bytumor-unique scattering phenomena (wherein the polarization,side-scatter, forward-scatter returns or tumorinduced resonant effectsare utilized). Additionally, "active methods", such as time-gating orpulse-compression methods (sometimes employed in modern radar systems)may also be used.

Another feature of this invention is the use of a stepped frequencytechnique to develop a synthetic time domain response. As opposed toapplying a large amplitude short duration pulse and then usingtime-gating or the use of swept frequency FM "Chirp" radar pulsecompression methods, the stepped or swept frequency input impedancemethod can be more easily implemented. The dwell time at each frequencycan be adjusted to give adequate signal-to-noise ratios, digitalprocessing and control can be used, and the hardware needed to implementthis method is available.

Another feature of this invention is the use of confocal techniques,where the focal point of the illumination and the focal point of thecollection system are nearly the same point in the breast tissue. Sucharrangements suppress the effects of incidental sources of scatteringthat might occur at locations outside the common focal point.

Another feature is the combined use of the confocal method with thestepped frequency synthetic time domain method, especially for detectinganomalies at depth. On one hand, the confocal method is most effectiveat shallow depths, and loses its ability to suppress incidentalscattering for deeper tumors. The synthetic time domain method, if usedseparately from the confocal arrangement with more commonly availableantennas, will generate back scatter from sources over a wide area.Scatter from such incidental sources could mask the desired returns fromany tumor. However, the combined use will provide more benefit thanwould be suggested by the performance of each subsystem separately. Theconfocal method suppresses clutter sources (incidental scattering) thatare transverse to the direction of propagation and the time domainsystem suppresses clutter sources in the longitudinal direction.

Another feature of the invention is convenience to conduct screening. Asopposed to other microwave methods that require access to nearly allsides of the breast, the method noted here needs access to only oneside.

Another feature of the invention is that it can provide non-hazardousscreening functions, such that breasts, over time, can be compared todetect abnormalities that would not otherwise be possible with anionizing approach or more expensive methods, such as MRI.

One version of the invention includes a quasi holographic techniquewherein the amplitude and phase of the scattered returns are compared toa reference signal and subsequently used to form some type of threedimensional display. A modified interferometer technique can be used todo this. The interferometer provides 3-D displays of the backscatteredpower and the cumulative phase shift of the returns with respect to areference point.

BRIEF DESCRIPTION OF THE DRAWINGS

The following figures are used to explain the concepts and design of thebreast cancer detection system of the invention:

FIG. 1A, is a conceptual view an active millimeter wave breast cancerdetection system, with a patient;

FIG. 1B illustrates displays for plural focal lengths, with thegeneralized system of FIG. 1A;

FIG. 2 is a simplified block diagram that illustrates the principalfunctions of a mmw breast cancer detection system constructed inaccordance with the invention;

FIG. 3 is a graph of relative dielectric constants of muscle, fat,breast tissue and breast cancer as reported by various investigators;

FIG. 4 is a graph of conductivity of muscle, fat, breast tissue andbreast cancers as reported by various investigators;

FIG. 5 is a graph of attenuation, wavelength, and depth of penetrationin normal breast tissue as a function of frequency, based on the datapresented in FIGS. 3 and 4;

FIG. 6 is a block diagram of a mmw breast cancer detection system,according to the invention, that employs a "passive" signal separationtechnique in combination with a conventional heterodyne receiver todetect tumor-scattered returns;

FIG. 7 is a block diagram of a breast cancer detection system, againaccording to the invention, that employs phase coherent detection;

FIG. 8 is a diagram of resolution (or spot size) and depth of focus asfunctions of the diameter of an aperture or lens and of wavelength;

FIG. 9 shows Snell's Law effects that illustrate quasi-opticalpropagation from a medium with a low relative dielectric constant into amedium with a very high relative dielectric constant;

FIG. 10 is a schematic cross-sectional illustration of a large apertureilluminator in the form of an ellipsoidal reflector in combination witha boot that contains a material having relative dielectric propertiessimilar to those for normal breast tissues;

FIG. 11 is a graph that shows the backscatter cross-section of aperfectly conducting sphere normalized to the cross sectional area ofthe sphere as a function of the circumference-to-wavelength ratio;

FIG. 12 presents a simplified block diagram of another detection system,according to the invention, that employs an "active" or time domaintechnique to help extract power scattered by a tumor from applied orimpinging mmw power;

FIG. 13 illustrates an array of double ridged wave guides that can beused to replace the ellipsoidal reflector;

FIG. 14 presents a simplified functional block diagram on how the phasedarray can be controlled to position the focal point without the need formechanical scanning;

FIG. 15 shows how the a wave guide like that of FIG. 13 can bepositioned directly on the breast for screening purposes; and

FIG. 16 illustrates how forward scattering can be sensed by a confocalarrangement wherein the focal point of the receiving array tracks thefocal point of the illuminating array.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The use of electromagnetic microwave or millimeter waves offers severaladvantages over x-ray mammography in detecting incipient breast cancers.(To simplify this discussion both microwave and millimeter wavelengthregimes will be referred to as mm waves, or mmw.) Non-ionizingelectromagnetic systems can be operated at sufficiently low levels so asto preclude biological hazards. A contrast ratio of the order of 20:1 ispotentially usable for mm waves in tissue, whereas there is less than afew per cent range of densities for X-rays for soft tissue. Thetissue-mm wave interaction also 5 exhibits additional phenomena that canbe drawn upon to enhance the performance. For example, when the diameterof a highly conducting sphere (e.g., an incipient cancer) is of theorder of a wavelength in the breast tissue, a resonance effect occursthat increases the effective scattering cross-section of the tumor. Ifthe tumor is non-spherical, then the polarization of the scattered wavesmay be different than that of the impinging waveform. In some cases,side-scattered or forward scattered energy can also be utilized. Forpurposes of this specification, tissue-mm waves are defined in terms ofwavelength in a medium having a dielectric constant like that of breasttissue, not air. Thus, the operating frequency for an electromagneticwave source used in the inventive system is preferably in the range ofthree to ninety GHz.

Other than the use of millimeter wave and microwave thermography todetect breast cancers, there has been little activity toward use of suchmm wave approaches to detect breast cancers. As noted earlier, some ofthe problems that have to be overcome are formidable. First, simplyflooding the torso of a female with mmw energy introduces numerousproblems. How does one single out the scattered return from a threemillimeter circumference tumor out of the immensely larger scatteredreturns from the torso? How is the defocusing effect of the air-skininterface overcome? Is the breast tissue sufficiently transparent, atmmw frequencies, to propagate energy into and out of the breast? Are thedielectric properties of the tumors sufficiently different from normalbreast tissue for effective detection of small (e.g., three mmcircumference) incipient cancers?

To understand the invention and its novel features, the basic conceptwill first be briefly described. Next, the ability of the millimeterwave electromagnetic energy to penetrate normal breast tissues will bedemonstrated. Then, the special equipment and operating conditions willbe described to realize the needed high resolution simultaneously withgood penetration.

FIGS. 1A and lB illustrate the basic concepts. FIG. 1A illustrates, on aconceptual basis, possible prototype equipment. The patient 21 arrangesone of her breasts 22 to contact an illuminator 23 as shown. Mm wavesare generated within the equipment housing 24. These mm waves are thenpropagated into the selected breast 22 as a refracted or reflectedelectromagnetic mm wave that is focused at a predetermined point orvolume (voxel) within the breast. This is done by means of a uniquecombination of an interface and focusing apparatus, as describedhereinafter. Further apparatus is used to cause the focal point of thebeam to scan different small volumes or voxels within the breast. Whenthis happens, the scattered mmw energy from any tumor present in thebreast becomes much larger that other scattering sources, since thedielectric properties of a tumor are radically different than that ofthe breast tissue. The scattered returns may be collected asbackscattered power via the same interface and focusing apparatus thatis used to propagate the mmw power into the breast. The collected powercan then be processed by either analog or digital methods to form animage of the tumor.

A stepped FM sweep similar to pulse compression in "Chirp" radar tosynthesize a time domain response to isolate shallow from in-depthscattering can be used to mitigate the effects of heterogeneity in thedielectric characteristic of the breast. The functional goal of thecombined confocal and time-domain features is to isolate the returnsfrom tumors from spurious returns generated by heterogeneity in adjacentnormal tissues.

Electromagnetic waves in the mm wave region, in combination with theshape and dielectric properties of tumors in the breast, offeradditional methods to detect the presence of a tumor beyond using justsimple back, forward, or side scatter. In some cases, the tumor canexhibit both internal and external electromagnetic resonances which areunique to its presence. Such resonances can be detected by varying themmw frequency, by observing changes in polarization, by observingtransient responses to an impulse function, or by noting changes in theratio of the forward, side, backscattered and spurious returns. If thegeometry of the tumor is asymmetrical, then the plane of polarization ofthe scattered energy may change; this change can be used to confirm thepresence of a small tumor.

The amount of collected backscattered energy and its accumulated phaseshift (or time of flight or round trip time delay) can be presented in a3-D format, as shown generally in FIG. 1B. For illustrative purposes, itis assumed that the impinging energy can be selectively and sharplyfocused into three vertical planes that are parallel to the patient'schest, wherein the x-y planes at maximum depth 25, medium depth 26, andshallow depth 27 are shown. The three coordinates show the backscatterreturns 28, the "x" coordinate 30 and the "y" coordinate 29. Smallvertical lines 31 are shown for numerous combinations of x and ycoordinates.

The amplitudes or heights of most of these lines 31 are proportional tothe non-target returns that can arise from, for example, the tissuesthat surround the rib cage. Note that a very large return 32 exists inthe center of the medium depth display 26. This large return 32 isassumed to arise from a tumor that is at the focus of the impingingenergy in the medium depth plane 26. In the center of the Shallow plane27 there is a somewhat smaller response 33 caused by the tumorintercepting and only scattering a small portion of the impinging beam.Note that in the center of the deeper plane 25, the return 34 is smallerbecause, it is assumed, the focused energy has largely been scattered bythe tumor in the medium depth plane 26 before it arrives at the centerof the deeper plane 25.

FIG. 2 presents a block diagram that illustrates the principal functionsof the mmw breast cancer detection and imaging apparatus. Microwave ormillimeter wave power is generated by a mmw power generator 41. Thispower is supplied to a power and signal director 43 via a cable orwaveguide 42. The power output 44 from the director 43 flows to anilluminator 47 via a waveguide 46. The illuminator 47, via a scanningplate 49 of dielectric material, causes the power to be focused at apoint 62 within the breast 22. A scan control 48, via a mechanicalconnection 50, causes the illuminator 47 to move along the plate 49 in apredetermined scanning pattern. When the focus of the power encounters atumor 61, backscattered power 45 is collected by illuminator 47 and isreturned, via waveguide 46, to the power and signal director 43.

To prevent the applied power from swamping or masking returns from thetumor 61, means must be included to extract the desired return or signalfrom the tumor from the applied power. The initial function of thedirector 43 is to direct the mmw power output 44 from the generator 41to the focusing illuminator 47 through the cable or waveguide 46. Thedirector 43 also is employed to extract the tumor-scattered returns 45that are collected by the focusing illuminator 47. The extracted returns45 from the power and signal director 43 are applied to a mmw receiver53 via a cable or waveguide 52.

The requisite directing action of the director 43 can be realized byseveral "passive" means, such as a balanced bridge circuit or magic tee,a directional coupler, or a circulator (see Ramo et al. (1965) Fieldsand Waves in Communication Electronics, John Wiley and Sons, New York,sections 11.17, 11.8 and 9.16). "Active" means of separating the appliedpower 44 from the tumor-scattered power 45 are possible in the timedomain. For example, very short duration pulses of mmw energy can beapplied and the returns separated by time gating methods. Other "active"methods currently employed in some modern radar systems can be used,such as pulse compression, chirp or frequency modulation radar; seeSkolnik, Introduction to Modern Radar Systems, McGraw-Hill (1980).

The focusing illuminator 47 of FIG. 2 has several functions. Oneprincipal function is to focus the applied power at a predeterminedpoint within the breast 22. Another principal function is to conditionthe spatial distribution of dielectric material to enhance resolution.Yet another function is to suppress dielectric and electrical interfacereflections that could mask the scattered returns from a possible tumor61 in the breast 22 of the patient 21. These functions may be done bymatching the wave propagation characteristic or dielectric constant ofthe illuminator 47 and the scanning plate 49 to that of the breast 22and also matching the electrical interface between the cable/waveguide46 and the focusing illuminator 47 by means of electrical matchingnetworks, such as the mmw equivalent of a "pi" or "tee" or "L" network.

The scan control 48 controls the position of the focal point 62 of theilluminator 47 via a mechanical connection 50 that slides theilluminator 47 over the scanning plate 49 in a predetermined scanningpattern. This action provides x and y positioning of the focal point.Alternatively, other techniques may be used in conjunction with the scancontrol 48 to create an apparent focal point, such as by means of phasedarrays or synthetic aperture methods.

The scattered returns 45 from a possible tumor or other scatteringsources are applied to the mmw receiver 53 via the wave guide 52 and thepower and signal director 43. This mmw receiver 53 may be a conventionalheterodyne receiver that provides an output proportional to the powerreceived. Alternatively, using a waveguide 58 connected from the mmwgenerator 41 to the receiver 53, a reference signal can be compared withthe return signal 45 to develop composite phase and amplitude data.

The outputs from the mmw receiver 53 are supplied, via a cable 54, to asignal processing and display unit 55. This unit 55, with a furtherinput from the scan control 48 via a cable 51, processes the receiveddata into a suitable display, such as illustrated in FIG. 1B.

In the case of screening for breast cancer, the performance requirementscan be relaxed, since the detection of an abnormality is the real goal.This can be done by comparing the returns from one breast with theother. In addition, year-to-year examination data can be compared. Asshown in FIG. 2, information via cable bundle 70 on the returns fromeach breast can be compared; see block 71. Cable bundle 72 carriessimilar data for storage and subsequent comparison via block 73 aftereach yearly examination.

Various known comparison techniques can be used for this purpose. Forexample, a transparency of a positive image taken of the breast at onetime may be overlain with a transparency of a negative image taken at alater time. Any significant return on the positive is presented as avery light gray area in the refrence-gray background and the returntaken a later time is displayed as a dark area in the reference-graybackground. If no change has occurred, the light gray area on thepositive and the dark gray area on the negative will tend to cancel andresult in a nearly reference-gray density. If some change has occurredin a specific area, the images will not cancel in this region, and theabnormality will be indicated as either a darker or lighter region inthe reference-gray background. A similar process can be done digitally,and the difference displayed visually in a two dimensional display for agiven "slice" or depth into the breast.

FIGS. 3, 4 and 5 provide data that demonstrate that non-lactating breasttissue has different dielectric properties than either tumors or muscletissues. Moreover, the attenuation of mm waves in such breast tissue isnot excessive in the 5 to 15 GHz region and hence permits reasonableoperating conditions for "passive" power and signal directors.Additional attenuation can be tolerated by the use of "active" power andsignal directors such that operation up to sixty GHz is possible.

FIG. 3 summarizes data on relative permittivity, scale 101, as afunction of frequency, scale 102. These data demonstrate that therelative dielectric properties of low-water-content tissues and normalbreast tissues are significantly lower than for high-water-contenttissues and tumors, either human or non-human. The low-water contentdata for curve 103 were developed by Chaudhary (1984) for human breasttumors. Johnson (1972) developed the data for curve 105 for fat, boneand low-water content tissue. Edrich (1976) generated the data forcattle fat, shown in the curve 107. Burdette (1986) generated in vivodata for canine fat, illustrated in a curve 109. The high-water-contentdata for another curve 104 was developed by Chaudhary (1984) for humanbreast tumors. Johnson (1972) developed data for muscle and high watercontent tissues, shown in a curve 106. Rogers (1983) generated the data,shown in a curve 108, for mouse tumors. Edrich (1986) collected data forcanine muscle, illustrated in a curve 110. Burdette (1986) provided invivo data, shown in curve 112, for canine muscle tissue. Note that inthe case of muscle or tumor tissues, the relative dielectric constant isof the order of forty or more, depending on the frequency. In the caseof low-water-content tissues, such as breast or fat, the dielectricconstant is in the order of five to ten, as measured for in vitrostudies. The in vivo measurements of Burdette (1986), shown in curves109 and 112, show an approximate increase by a factor of two in therelative permittivity over the data developed by Johnson (1972), curves105 and 106. The in vitro breast tissue measurements by Chaudhary(1984), curves 103 and 104, fall somewhat in between the in vitro valuesdeveloped by Johnson (1972) and the in vivo measurements of Burdette(1986).

FIG. 4 presents similar data on the conductivity of both low andhigh-water-content tissues. The conductivity in mhos/meter, scale 121,is the ordinate and the frequency, curve 122, is the abscissa. Thelow-water-content tissues are human breast tissues, shown in a curve 123derived from Chaudhary (1984). The low-water-content fat and bone of thecurve 125 is from Johnson (1972). Cattle fat, shown in a curve 127 isfrom Edrich (1986). The high-water-content tissues of the curve 124 arehuman breast tumors, data by Chaudhary (1984). High-water-content muscletissue is in a curve 126, data by Johnson (1972). Mouse tumors, shown ina curve 128, are from Rogers (1983). Rat muscle data for a curve 130 isderived from Edrich (1986). Canine fat data are presented in a curve 131from Burdette (1980), and canine muscle data in a curve 132 taken fromBurdette 1980). Note that conductivity, as a function of frequency,tends to increase substantially above 6 GHz and that the 40 to 90 GHzmeasurements of Edrich (curves 127 and 130) tend to fall in line withthe trends established by measurement made up to 10 GHz.

Based on the data presented in FIGS. 3 and 4, FIG. 5 shows the depth ofpenetration 140, wavelength 142, and attenuation 144 as a function offrequency 146 for the propagation of millimeter waves in non-lactatingbreast tissue. Above ten GHz, some uncertainty associated with the trendextrapolation is suggested by the range of possible values of thepenetration depth 140 or attenuation 144. A value of nine was used forthe relative dielectric constant and the extrapolated values ofChaudhary (relative to the data developed by Johnson) from FIG. 4 wereused for the conductivity. From these data, it is seen that the breasttissue behaves as a lossy dielectric for frequencies substantiallyexceeding five GHz, wherein ω=2πF and .di-elect cons.=.di-electcons._(O) .di-elect cons._(r) (permittivity of free space)×(relativedielectric constant), σ is the conductivity, μ is the permeability, f isthe frequency, λ is the wavelength, and δ is the depth of penetration(see Ramo (1965) page 334 Sec. 6.05).

Since ω.di-elect cons.>>σ, the approximate lossy dielectric equationsare as follows:

    λ=(μ.di-elect cons.).sup.-1/2                    (1)

    δ=2 σ(μ.di-elect cons.).sup.1/2 !.sup.-     (2)

This defines the generic feasibility of the system to be describedhereinafter. There are two requirements that must be met. First, thetotal path loss attenuation (in and out) should be substantially lessthan the dynamic range, typically in the order of 100 dB, wherein thedynamic range is defined in dB as equal to: 10 log (largest signalpower)/(smallest detectable signal power)!. Second, the wavelength inthe irradiation apparatus (illuminator 47) and in the breast of thepatient should be sufficiently small so that small tumors can beresolved. This, for the system discussed here, requires that,preferably, the wavelength in illuminator 47 and in the breast tissueshould not exceed two or three times the circumference of the smallesttumor. If an operating frequency of 15 GHz is chosen for a passive powerand signal detector, it is seen that the path loss is about 5 dB/cm, or50 dB total path loss, in and out, for a 5 cm path length. Thewavelength at 15 GHz is about 0.6 cm, which is about equal to thediameter of the smaller tumors.

FIG. 6 illustrates a functional block diagram of a microwave breastcancer detection and imaging system 200 that employs a conventionalheterodyne receiver. System 200 comprises the following subsystems: amillimeter power generator subsystem 241, a passive power and signaldirector 243, a focusing illuminator subsystem 228, a heterodynereceiver 253 employed for signal detection, a scanner control 248, and asignal processing and display subsystem 255.

Electromagnetic wave energy flows, via the power and signal director243, from the power generation subsystem 241 to the illuminatorsubsystem 228. The illuminator subsystem 228 comprises three majorparts: a beam focusing apparatus 247, a matching network 226, and adielectric equalizing interface 249. The focusing apparatus 247 of theilluminator 228 focuses the energy into a small point 262 within thebreast of the patient 259. The scanner 248, through a mechanicalconnection 250, controls the location of the focal point 262 in threedimensions (3-D) such that the focal point 262 is progressivelypositioned into each voxel (smallest volume element) of the breast underconsideration. When the focal point encounters a tumor 261, thescattered returns are substantially increased, since the electricalpermeability and conductivity of the tumor is greater than similarparameters for breast tissue. The scattered returns are collected by theilluminator 247 of subsystem 228 and then the scattered power (arrow245) is supplied via the matching network 226 and the power and signaldirector 243 to the detection subsystem 253.

The scattered power 245 is separated from the impinging power 244(supplied to the illuminator) by means of a circulator 224 within thepower and signal director 243. A discussion of each of theaforementioned subsystems follows.

The power generation and control subsystem 241 is comprised of twofunctional blocks: an electromagnetic wave power source 220 connectedvia a cable 221 to an isolator and power splitter 222. This, in turn, isconnected, via a cable 242, to the power input port 225 of thecirculator 224 in the power and signal director 243. The power outputand backscattered input port 227 of the circulator 224 is connected, viaa cable 246, to the matching network 226 at the input of the illuminatorsubsystem 228. The output port 234 of the circulator 224 is connected tothe signal processor subsystem via a cable 252.

Many of the functions of these components are obvious. Theisolator/power splitter 222 electrically isolates the power source 220from any load variations that might be introduced by the circulator 224,the matching network 226, or other components of the illuminatorsubsystem 228. The function of the circulator 224 is to extract thebackscattered returns from the applied power. Otherwise, the high levelof the power applied to the illuminator subsystem 228 would tend to maskthe desired scattered returns. Thus, the electromagnetic input signalinjected into port 225 is directed out of port 227 and thence to thematching network 226. The backscattered returns (from the matchingnetwork 226) that are applied to port 227 appear at port 234, whereinthe amplitude of the applied power is greatly suppressed. The purpose ofthe matching network 226 in subsystem 228 is to suppress reflectionsthat might take place at the interfaces of different dielectricmaterials or where some wave impedance discontinuity occurs in theilluminator subsystem 228.

The performance requirements for the signal detection system 253 are nottoo stringent. The simplest version may use a simple heterodyne receiveras an RF voltmeter to measure the output of the circulator 224 at port234. A reference signal from the power generation subsystem 241 can besupplied to the hetrodyne receiver 253 via a conductor 258 to stabilizethe local oscillators in the receiver.

Other versions of the invention, such as the system 300 shown in FIG. 7,offer additional signal processing options. The system 300 of FIG. 7illustrates the use of two synchronous receivers or detectors in amodification of the system of FIG. 6 in which only the signal detectionsubsystem is changed, with subsystem 253 of FIG. 6 replaced by a dualsubsystem 353 that includes two hybrid tee synchroneous detectors 361and 362. The power splitter 222 (FIG. 6) provides a reference signal, onconductor 258 (FIG. 7) to system 353 as well as to the power and signaldirector 243. Each of the hybrid tee devices 361 and 362 forms a productbetween the applied input signal and the composite backscatteredreturns. However, one of the reference waveforms is shifted ninetydegrees with respect to the other reference waveform; the followingrelationships result, where:

ω is the angular frequency of the millimeter waves;

θ is an arbitrary reference fixed phase angle;

λ is the wavelength;

χ is the path length from the scatterer to the hybrid tee;

β is the propagation phase constant and β=2π/λ;

χβ is the accumulated phase shift.

The output from each of the hybrid tees 361,362 is the product of thereturned, scattered waveform and the reference waveform. Consideringjust the low frequency components of such products, the output of eachof the hybrid tees is as follows:

    A cos  ωt+θ-2χβ! cos  ωt+θ!=A/2  cos (-2χβ!+high freq. component                      (3)

    A cos  ωt+θ-2χβ!cos  ωt+θ-π/2!=A/2 sin (-2χβ)!+high freq. componet                      (4)

The outputs can be defined as an in phase "I" vector component and aquadrature or "Q" vector component. These are combined as vectors so thephase angle of the combined vector becomes tan=¹ (2χβ). Typically, whenno tumor is at the focal point 262 (FIG. 6), the backscatter from thechest tissue-lung interface can be assumed to form the zero referencedistance for χ. During scanning, the focal point may begin to encountera tumor that is spaced a few or more millimeter wavelengths away fromthe chest muscles. When this happens, the effective distance χprogressively decreases, thereby causing the equivalent phase angle torotate counter-clockwise. The number of rotations can be counted todevelop the total accumulated phase angle change. The aboverelationships can be manipulated to present an accumulated path delaypresentation that is responsive to the approximate distance of thescatterer from the illuminator. Such an option can be valuable inconfirming the presence of a weak scatterer and can provide confirminglocation data for a strong scatter.

The dual receiver system depicted in FIG. 7 draws the referencewaveforms from the isolator-power splitter 222 via cable 258. Anattenuator-power splitter 339 is used to reduce the amplitude of thewaveform presented to the two phase shifters 340 and 341 via appropriatecables or other conductors 352 and 351. The output waveform of phaseshifter 341 is advanced or retarded ninety degrees relative to phaseshifter 340 to provide the desired quadrature relationship. Thequadrature reference waveforms from circuits 340 and 341 are applied,via cables 342 and 343, to the hybrid tees 362 and 361, respectively.The output of port 234 of the circulator 224 supplies the power from thebackscattered returns, via cable 252, to the power splitter-isolator335. This circuit 335 diverts the return signal equally into cables 336and 337, thus supplying the backscattered signals to the hybrid tees 361and 362. These tees 361 and 362 each form a product between thereference waveform (from conductors 343 and 342, respectively) and thebackscattered signals (on lines 336 and 337, respectively). The lowfrequency output from these two devices 361 and 362, on cables 345 and346, provides critical inputs to the signal processor and displaysubsystem 255 (FIG. 6). Other variations of the above technique may beused to improve the signal-to-noise ratio, such as modulating thereference waveforms with another frequency well above the highestfrequency of interest in the detected backscattered return. This removesthe output signal well away from the troublesome shot noise that occursat very low frequencies.

The scanner control subsystems 48 and 248 (FIGS. 2 and control how thebreast of the patient is scanned. In the case of the prototype system ofFIG. 2, scanner control 48 controls the x and the y positions of anellipsoidal reflecting antenna 47, which may be the antenna 170 shown inFIG. 10. Several antennas of different focal lengths may be used toaccess the location of a tumor. The scanner control (48 or 248) ismechanically connected to the illuminator (47 or 247) and to the signalprocessing unit (55 or 255) in each of the described systems of FIGS. 2and 6. Phased arrays (not shown) could be used instead of a mechanicallypositioned illuminator to realize approximately the same scanningperformance. Other methods, particularly techniques that synthesizelarge aperture antennas, could also be used.

In any of the described systems the signal processing and displaysubsystem (e.g., subsystem 255 in FIG. 6) can employ any number ofprocessing or display methods so as to suitably display the scatterreturns. It should be noted that since the scatter waveforms arereferenced to the initial unperturbed electromagnetic wave illumination,quasi-holographic processing techniques can be considered.

FIG. 8 defines the parameters needed to determine the spot size,including the diameter of the aperture D, the focal distance R, the spotdiameter d, and the wavelength λ of the millimeter wave in the media.See Kay (1966) and Smith (1966) for more complete development ofrelationships. Here, the spot size becomes:

    d=2Rλ/D                                             (5)

As was noted earlier in regard to FIG. 5, reasonable penetration lossesof about five dB/cm occur for wavelengths of the order of six mm. Thus,if tumors in the order of three mm in circumference are to be resolved,the beam width or spot size should not exceed the tumor circumference bymuch more than a factor of three. Preferably, for improved spatialresolution, the wavelength should not exceed the tumor circumference bya factor of three. To achieve a spot diameter of 6 mm, the ratio of thefocal distance R to the aperture diameter, D should be about 0.5.

Another design consideration is the depth of field Δ, as related to theaforementioned parameters and the apparent angle of resolution Φ. Thusthe depth of field becomes:

    Δ= R.sup.2 Φ!/ D±RΦ!, where Φ=d/R     (6)

Again, to obtain good spatial discrimination, the focal distance Rshould be small compared to the aperture diameter D.

However, short focal lengths cannot be easily developed if thedielectric constant between the media that form an interface are greatlydifferent. This would be the case if an attempt is made to propagatemillimeter wave power in air and thence into the breast. As seen in FIG.3, the dielectric constant of breast tissue is of the order of nine, andsuch a large value (relative to a value of one for air) causessubstantial reflection of the incident power at the air-breastinterface. More importantly, the apparent R/D ratio is reduced; that canlead to a radical increase in the spot size. This is best seen byreferring to FIG. 9 (see Ramo (1965) p. 358 Sec. 6.13 for more completebackground). Here the incident ray 151 impinges on a dielectricinterface 150. This produces a reflected wave 153 and a penetrating wave155. The angle of incidence 157 must equal the angle of reflection 159.Also, Snell's Law must be satisfied where angle 161 is the angle of thetransmitted penetrating wave 155 with respect to a normal to the planeof incidence 150 and .di-elect cons.₁ and .di-elect cons.₂ are thedielectric constants for air and for the breast tissue, respectively:

     sin 161!/ sin 157!= .di-elect cons..sub.1 /.di-elect cons..sub.2 !.sup.1/2(7)

This interface reduction in the angle 161 of the transmitted penetrationwave 155 will tend to increase the apparent R/D ratio (focal distance toaperture size ratio). For example, assume that a focused mm wave frontwith an R/D ratio of 0.5 in air impinges on a dielectric interface wherethe ratio of the relative dielectric constant of the second media to thefirst media is nine. Based on Snell's Law, the maximum value of angle161 can be no more than about fourteen degrees. This would increase theapparent focal distance from R to about 4R and increase the spotdiameter d by a factor of four.

FIG. 10 illustrates one way the defocusing and reflecting effect of thedielectric interfaces between a breast (or other tissue) and air can bemitigated. The mm wave power from a feed point 171 in a medium 175 isintroduced, via an interface medium 176, into the breast 177, with therespective relative dielectric constants .di-elect cons.₁, .di-electcons.₂ and .di-elect cons.₃ made approximately the same. Forillustrative purposes, an elliptical reflector 170 is chosen; reflector170 has an R/D ratio of about 0.5 that would produce a spot sizecomparable to the wavelength, which is of the order of 6 mm. Thiselliptical reflector 170 and related apparatus is designed to exhibittwo focal points: one at the feed point 171 and the other at a point 172in a voxel within the breast 177. An interface boot 180 is used tocontain a liquid or slurry 176 having a dielectric constant, .di-electcons.₂, that is contained by a boot wall 181, a thin, liquid-impermeablebrassiere 182, and a thin, solid dielectric sheet 183 that has the samerelative dielectric constant as .di-elect cons.₁. For orientationpurposes, the dash curve 178 represents the curve of the ellipse ofreflector 170 if it were to extend into the human body.

The thin brassiere 182 is caused to fit as closely as possible to thebreast 177 of the person under examination, as by withdrawing air fromthe voids 179. The breast is compressed into a relatively flat surfaceby the dielectric plate 183. This permits moving the focal point 172 bymoving the elliptical reflector 170 along the surface of the dielectricplate 183 to scan the breast. The focal point 172 may be moved upwardlyby increasing the thickness of the dielectric plate 183.

This arrangement assures that the mmw power that is applied to thebreast is focused in the voxels of interest. Future, the millimeter wavepower that is scattered from a possible tumor at the focal point 172returns to point 171 via paths that have equal time delay, thus allowingconstructive recombination of the scattered returns at the feed point171. On the other hand, power that is not intercepted by a tumor atfocal point 172 progresses on to the breast-lung interface 185 viaplural paths such as the path 186. A portion 187 of the unscattered wave186 progresses into the muscle and rib cage of the patient; anotherportion 189 is reflected. In order for these back-scattered or reflectedmuscle wall waves to add constructively at the feed point 171, the waveswould have to experience the same path lengths, in terms of integralmultiples of a wavelength, as for the paths of the waves scattered atthe focal point 172. For most of the unscattered waves that arereflected from the breast-lung-rib cage interface, constructive additionat the feed point 171 is quite unlikely. An arrangement as shown in FIG.10, therefore, suppresses unwanted returns or clutter from muscles andfrom the patient's rib cage relative to the returns that are scatteredat the focal point of interest, point 172.

Other dielectric anomalies 190, such as might arise from a blood vessel,will also scatter the incident millimeter wave power. However, like theclutter returns from the muscle-rib cage, such returns will besuppressed in amplitude or even omitted, relative to the returns fromthe voxel containing the focal point 172. Furthermore, since many raypaths are used, any minor and random variations of the dielectricconstant will tend to be averaged out.

Other methods of discriminating the presence of a tumor from the clutterscatter returns are possible. FIG. 11 illustrates one such phenomenon, aresonant enhancement of the scattering cross section, σ, that occurswhen the circumference of a highly conductive spherical scatterer equalsthe wavelength in the media around the sphere. The cross-sectionrelative to the projected area of the sphere is shown as the ordinate401. The abscissa 402 is the ratio of the circumference to thewavelength. The curve 403 illustrates how the cross section is enhancedby the resonant scattering that takes place between the Rayliegh region404 and the Mie (resonance) region 405. In addition, internal resonanceswithin the tumor are possible. Despite the higher conductivity withinthe tumor, the material within the tumor still behaves as a lossydielectric because of the very large value of its relative dielectricconstant. Therefore, internal resonances within the very high dielectricconstant tumor can be considered to occur where the internal dimensionsare in the order of a one half wave length--or about 3 mm for anoperating frequency of 10 GHz. Such resonances can generate a uniqueresponse, quite different from those generated from dielectricinterfaces or other sources of clutter. Such resonant responses can beobserved by sweeping the frequency over a wide bandwidth or exciting thebreast tissues with a very short duration mmw pulse and then observingthe resonant quasi-sinusoidal decay.

The foregoing description was aimed primarily at a prototype suitablefor clinical evaluations. A commercial version may avoid the use of theboot and the elliptical reflector (FIG. 10). For example, arrays of 6 mmrectangular waveguides may be filled with dielectric material that has arelative dielectric constant similar to that of the breast, roughly inthe order of nine to sixteen. The rectangular guide in the TE₀₁ modereadily propagates 10 to 15 GHz frequencies. Each of the guides would bepositioned in contact with the breast, directly or through a very thinmaterial used for one of several standard brassieres. The phase angle ofthe power presented to the guide-breast contact point would be identicalto the phase angle of each ray path, some of which paths are shown inFIG. 10. For example, at positions 192 and 194 of FIG. 10, the phaseangle of the power at each of the waveguide exits would be the same asthe phase angle for each of ray paths 191 and 193 respectively. Onehundred such 6 mm guides could be positioned in a 60 mm by 60 mm flatfaced rectangular array to replace the egg-shaped elliptical antenna 170shown in FIG. 10. Given a computer-aided ability to adjust the phase ofthe output power at each of the guides, the position of each of theguides could be made to fit the contours of many different breasts. Theadvantage of this approach would be to permit mass screening, but at theexpense of a rather complex piece of equipment. The use ofcomputer-controlled functions and data analysis does make such anapproach feasible.

The foregoing can be better understood by referring to FIG. 13, whichshows a nine aperture wave guide module 450. Nine double-ridged waveguides 451 are used. Each of these are filled with a dielectric material452 that approximates the relative dielectric constant of normal breasttissue. In the case of a screening system, only four wave guideapertures might be used. The combination of modules pressed against thebreast in place of the dielectric plate 183 of FIG. 10. By properphasing of the signals to each of the wave guides, focal point can bepositioned within the breast without the need for mechanical movement.

FIG. 14 illustrates one way that the phase or timing of the signalapplied to the wave guides may be controlled to position the focal pointin a medium 418 which contains the aperture antennas 410A, 410B, 410Cand 410D and focal points 412 and 413. A source 400 of microwave powerapplies equiphased power via wave guides 401 and 402 to two powersplitters 403 and 404. The outputs of the splitters are applied, viawave guides 405A, 405B, 405C and 405D, to the circulators or directionalcouplers 406A, 406B, 406C and 406D. The forward power through thesedevices is transferred via the guides 407A, 407B, 407C and 407D to thevariable time delay or phase control devices 408A, 408B, 408C and 408D.The return power is transferred via wave guides 416A, 416B, 416C and416D to a subsystem 417 that collects the returns in a format suitablefor additional processing by subsystem 255 of FIG. 6. The time delay ineach device 408 may be controlled by changing the magnetic field biasapplied to a ferrite element within each of the devices. Such bias maybe supplied via the cables 414A, 414B, 414C and 414D from the time delaycontrol subsystem 415. The outputs from wave guides 405 are controlledby the signal processing and display subsystem 255 of FIG. 6. Via waveguides 409A-409D, the time delayed or phased controlled power issupplied to the aperture antennas 410A-410D. A portion of the outputsfrom these apertures reaches the desired focal point 412 via pathways411A, 411B, 411C and 411D. At point 412, the phases of the rays shownare nearly identical.

Assuming a time delay of t₁, t₂, etc. for each of the delay controlelements 408 and path lengths (411) d₁, d₂, etc., then t₁ +d₁ /v=d₄ /vfor constructive addition where v is the velocity of propagation in themedium 418. To meet this requirement, t₁ =(d₄ -d₁)/v. Other time delayscan be calculated in the same way.

Other methods of control are possible by controlling the phase of thesignals applied to each aperture instead of by the timing devices. Inthis case, the relative phase between the signals applied to apertures411C and 411D can be redefined by noting the following, where ω is theradian frequency 2 πf! and θ₁₂ is the phase difference between 411C and411D, such that θ₁₂ =ω t₂ -t₁ !.

The confocal arrangement permits the scattered signals to return by thesame pathways as the applied wave form. These signals are collected bythe aperture antennas 410 and progress back through the time delaydevices 408 to the circulators 406. These, in turn supply data on thescattered returns to subsystem 417.

FIG. 15 illustrates an alternate method of scanning the interior of thebreast. The microwave generation and signal recovery system 460 suppliespower to an array of mechanically positionable wave guides 461A, 461B,461C and 461D. Waveguides 461 are designed to slide into guides 462 sothat the distal end of guides 461 is in intimate contact with the breast463 on the chest 464 of an examination subject. As noted before, theposition of each guide can be measured and this information can be usedto calculate the timing or phase data needed to position the focal pointthroughout the breast. This method has the advantage that an interfaceplate such as 183 of FIG. 10 is not needed to compress the breast 463,FIG. 15 and that better and more reproductible contact with the breastcan be made.

Dielectric materials in solid form are available with relativedielectric constants that can exceed 100, that have relative low losses,and that can be machined. These materials can be used to fill theelliptical reflector or the waveguides (FIG. 10). Liquid or slurry-likedielectrics with both low losses and dielectric constants at 10 GHz inthe order of ten may not be readily available. However, some liquids,such as the silicones, have dielectric constants of nearly three. Suchoils could be mixed with particles of materials that have dielectricconstants that exceed 30 to 50 to form a slurry exhibiting a dielectricconstant of the order of nine. Conversely, acetone has a dielectricconstant of 22 at ten GHz, and it could be mixed with a silicone oil toproduce a dielectric constant of nine for the mixture. Otherpossibilities exist, including emulsions and similar techniques thatallow suspensions of one liquid in another or suspensions of particlesin a liquid.

The aforementioned techniques are not limited to backscatter; they canbe modified or augmented to detect both side scatter and forwardscatter. One forward scatter approach could use a paddle-like sourceantenna and a paddle-like receiving antenna. At least one of these wouldfunction similar to the antenna illustrated in FIG. 10. For example, themore pendulous portion of a breast can be placed between the paddles.This would permit examination of a breast that is 100 mm thick with asystem optimized for a 50 mm penetration depth.

The above may be better understood by referring to FIG. 16. Here, thebreast 480 is positioned within two dielectric plates 472A and 472B. Acover plate 483 and gaskets 482, together with plates 472, form abox-like structure that surrounds breast 480. The relative dielectricconstant of the box wall material is similar to that of the breast. Athin plastic film 481 covers the portion of the breast that is not incontact with the box. The space between this plastic film and the breastis filled with a liquid that has the same dielectric constant as thenormal breast. A cable bundle 470 supplies microwave power to a seriesof waveguides 471 that are in a housing 479A on plate 472A. Uponexcitation, the power in these guides is timed or phased such that theray paths 474 come to a focal point 476. The time delays in the guides477 in the receiving assembly 479B are timed such that any scatteringoccuring at point 476 will add constructively. The outputs of theseguides are carried to the signal processing subsystem via a cable bundle471.

The aforementioned techniques may also be readily modified to detectChanges in the plane of polarization. For example, a square TE₀₁waveguide could inject a vertically polarized wave and respond to ahorizontally polarized wave. Similar arrangements could be used foreither side scatter or forward scatter polarization anomaly sensingarrangements.

Another variation would be to use the illuminator and focusingarrangements described above in combination with an "active" or timedomain method of separating the applied power from the scattered power.Other such active or time domain methods utilize a "chirp radar" toproduce added resolution in depth and additional clutter suppression. Ata center frequency of 15 GHz a chirp radar with a swept frequencybandwidth in the order of 5 GHz and with phase correction for thedielectric behavior of the breast tissue could produce range cellresolutions of the order of 10 millimeters. Alternatively, sequences ofvery short duration bursts of 15 to 25 GHz waveforms should also provideisolation of the applied power from the backscattered power by timegating techniques. Burst durations in the order of 100 picoseconds willprovide depth discrimination of the order of 10 to 20 millimeters. Thisadded discrimination would not only suppress the incident power, butalso could suppress backscatter returns from the different dielectricinterfaces, such as the muscles around the rib cage.

Active methods are of particular interest because these methods may befunctional with total path losses in excess of 100 dB. Such path lossesmight be difficult to overcome with a passive system, since it may bedifficult to reduce clutter levels below 50 to 70 dB the applied power.Since some of the clutter can be reduced by considering only the returnsin just one voxel, active systems might be viable over a wider dynamicrange. Also, shorter wavelengths with greater resolution can be used,since active systems can accept greater path losses, possibly as much asmight be experienced by a system with an operating frequency as high as60 GHz.

To illustrate how time domain separation can be realized in broad bandpulse systems, FIG. 12 presents a further possible system 500. Many ofthe components are similar to those noted in FIG. 6; they are notrepeated in FIG. 12. In system 500 the power and signal directionfunctions of the isolator of FIG. 6 have been augmented by a modulator540 and a source 541 of 100 to 200 picosecond pulses. In some systems,the isolator 224 may be omitted. A pulse from the pulse source 541, viacable 543, gates on the modulator 540. This causes the modulator togenerate a short burst of 15 GHz sine waves that is applied, via a cable242, to the input 225 of the circulator 224. Also, the pulse source 541supplies a timing pulse to a blanker-modulator circuit 542 and to asignal processor 515. The blanker-modulator 542 suppresses any leakageof the incident pulse through the isolator 224 from impinging on abroad-band receiver 516. The timing pulse, via the cable 544, is used bythe signal processor 515 to determine the spatial position of thedifferent scattered returns by noting the time of arrival of each of thereturns. For example, the more distant returns that might arise from thepatient's rib cage would be delayed the most. The combination of apulsed sine wave source and a blanking function (often called a transmitand receive function) essentially provides a power and signal directionfunction that could replace the circulator 224 function as illustratedin FIG. 6.

The pico-second-duration-pulse, time-domain system described for FIG. 12has some drawbacks that may be overcome by a stepped-frequency,synthetic-time domain method. For example, the noise level of the widebandwidths needed to accommodate such short duration pulses can be quitehigh. On the other hand, the stepped-frequency method can have long welltimes at each frequency, thereby reducing the bandwidth and noise levelfor the signal processing system. While the confocal system is apowerful tool to suppress the effect of some classes of heterogeneities,it may not provide sufficient discrimination at deep penetration depths.To mitigate this problem, a time gating technique can be used tosuppress scattered returns from shallow depths. Such a method may notethe presence of a weakly scattering tumor at depth. If the relativedielectric constants of the intervening material are uncertain, thegeometrical position of the tumor cannot be precisely determined. Whilethis uncertainty poses a difficulty for an imaging system, it should notaffect the viability of a screening system designed to detectabnormalities.

Swept frequency methods can be considered. For example, an FM Chirpradar method that has been used in weapons detection systems effectivelyseparates desired returns from those generated by systemdiscontinuities. A version of this would be attractive in conjunctionwith the confocal illumination method to separate the effects of nearsurface discontinuities or hetrogeneities from the returns at greaterdepth. Linear FM pulse compression radar (PCR) techniques might also beconsidered. These have been described by Jacobi (1986, reference sevenhereafter) for biological imaging applications. The theoreticalresolution of a PCR is given by ΔR=C/2B, where C is the is the velocityof propagation in the media, and B is bandwidth of the transmitted waveform. Assuming a mid-band frequency of 8 GHz, a 5 GHz sweep and a mediumwith a dielectric constant of nine, a range resolution of one cm isindicated. However, a 2.5 GHz sweep may be more readily realized andcould produce an in-tissue resolution of two cm. To realize thisperformance, the FM sweep must be highly linear, a pulse compressionfilter developed for this application and the dispersion effects of thedielectric compensated.

A stepped or swept frequency input impedance Fourier inversionalternative exists. This option transforms data developed from thefrequency domain measurements to the time domain via digital processing,thereby eliminating the need for a pulse compression filter. This can beimplemented by using either the confocal illuminator of FIG. 10 or thephased array of FIG. 14. The output signals from the circuit shown inFIG. 7 on lines 346 and 345 can be viewed as a complex input impedance,S(jω), at a radian frequency of ω(ω=2 πf) to the illuminator. As thefrequency is stepped from a low frequency to a higher frequency, thecomplex input impedance for each frequency is stored in a digitalcomputer. If the frequency is swept or stepped over a band similar tothat noted for the PCR system, similar spatial resolutions can berealized. Via digital processing, the complex input impedance data isconverted from the frequency domain to the time domain using inverseFourier transformation. The transformed data is then in the form of anamplitude vs. time response, similar to a radar A scope display, as ifan impulse or stepped function had been applied at port 234 of thecirculator 224 in FIG. 7. Initially, the returns from systemdiscontinuities, such as from connectors and the interface with theantenna in the illuminator, will be displayed. Then, the reflectionsfrom anomalies in the breast will be displayed, the reflections from thedeeper anomalies occurring at the longer times. The stepped frequencyoption offers the opportunity to include a standard correction at eachfrequency increment for a typical dispersion characteristic for normalbreast tissues and could also include compensation for other factorssuch as path loss or system dispersion in the ferrite phase shifters.Some of the more modern network analyzers include a built-in stepped orswept frequency to time domain processing option.

The underlying mathematical basis is as follows. The general Fouriertransformations are: ##EQU1## Where F(f) is the impulse or step responseS(jω) is the Fourier transformation of F(f)

ω=2 π f, is time.

Throughout the foregoing specification and in the appended claims theterms millimeter waves, or mm waves or mmw have been used to genericallyrepresent the wavelengths of the electromagnetic waves that propagate inthe human breast tissue. Since the relative dielectric constant of thebreast is in the order of 9 to 12, the free-space wave length will bereduced by a factor of three or more. Thus, the in-tissue wavelengthsover a frequency range of 3 to 60 GHz will range from about 30 mm to 1mm.

As opposed to certain microwave hypothermia cancer treatment technology,none of the technology presented here is intended to heat significantlyany portion of the breast. This requirement limits the power depositiondensity onto the surface of the breast to less than 10 milliwatts/cm²and the volumetric heating rate in any portion of the breast to lessthan 0.8 milliwatts per gram of tissue as averaged over a time period ofa few minutes. To further assure minimal thermal effects, the inputpower is to be turned off if the scanning system falters for any reason.

Other usages are as follows: The term impedance refers to the ratio ofthe voltage to the current or of the electric field to the magneticfield at a specified location. This term impedance is qualified as"electrical" or "wave" respectively, depending on whether voltages andcurrents or electromagnetic fields are concerned. The term wave guide isused in the generic sense and includes both cables and higher mode waveguides with just a single transverse field. The terms effective apertureand effective focal point are used in the generic sense whereinapertures and focal points can be created physically or synthetically(such as often used in synthetic aperture radar).

The effective focal point is not really a point but rather is definedhere as a region where the illuminating energy is most concentrated inthe breast. The effective focal point is further defined as the regionor volume where this energy concentration occurs as affected by theheterogeneity of dielectric characteristics of the normal breasttissues, the in-tissue wavelength, the size and distance of theilluminating globular aperture or the geometry and number of aperturesused in a phased array. The focal point positioning may be eithermechanical or electronic as in the case of a phased array.

The terms "detect" or "detection" are also used in the generic sense,and may mean simply indicating the presence of a tumor or more broadlyproviding data that permits imaging the location, size and geometry ofthe tumor. Detecting, identifying, imaging or locating a tumor alsomeans noting the presence of an abnormality. The terms "power and signaldirector" or "input power and signal separation" are also used in ageneric sense. Both passive and active techniques not only enhancedetection by suppressing the direct effects of impinging power waves,but also can reduce false signals or clutter. Such are introduced byimperfect matches between impedances or by non-tumor scattering sources,such as the breast/lung interface.

The following references are of utility in understanding the foregoingspecification:

Burdette, E. C., et. a1.(1980): In vivo measurement techniques fordetermining dielectric properties at VHF through microwave frequencies,IEEE Trans. on MTT, Vol MTT-28, No. 4 April, pp 414-427

Burdette, E. C., et. al. (1986): In situ permittivity at microwavefrequencies: perspective, techniques, results, medical applications ofmicrowave imaging, Medical Applications of Microwave Imaging, Larsen, L.E. and J. H. Jacobi, IEEE press pp 13-40

Chaudhary, S. S., et. al. (1984): Dielectric properties of normal andmalignant human breast tissues at radiowave and microwave frequencies,Indian Jr. of Biochemistry and biophysiscs, Vol. 21, Feb pp76-79

Edrich, J., et. al. (1976): Complex permittivity and penetration depthof muscle and fat tissues between 40 and 90 GHz, (1976) IEEE Trans. MTT,vol. MTT-24, May pp273-275.

Johnson, E. C., et. al. (1972): Nonionizing electromagnetic wave effectsin biological materials and systems, Proc. of the IEEE, Vol. 60, No. 6,June pp 694-695.

Kay, A. F. (1966): Millimeter wave antennas, Proc. of the IEEE, Vol. 54,No. 4, pp 641-647

Larsen, E. L. and J. H. Jacobi, Eds. (1986): Medical Applications ofMicrowave Imaging, IEEE press. Institute of Electrical and ElectronicEngineers, New York, pp. 138-147

Ramo, S., et. al. (1965): Fields and Waves in Communication Electronics,John Wiley and Sons, New York

Rogers, J. A., et. al. (1983): The dielectric properties of normal andtumor mouse tissue between 50 MHz and 10 GHz, British Jr. of Radiology,vol. 56, May, pp 335-338.

Smith, W. J., (1966): Modern Optical Engineering, Mc Graw-Hill, NewYork, N.Y.

I claim:
 1. A non-invasive system utilizing non-ionizing electromagneticmillimeter waves of minimal thermal heating capacity for detection of atumor in a breast, comprising:a generator for generating a non-ionizinghigh-frequency electromagnetic input wave; illumination means forcreating an effectively focussed non-ionizing electromagnetic beam fromthe input wave and directing said beam to impinge upon an effectivefocal point at a predetermined position within a breast; wave impedancematching means, in the illumination means, matching the impedance of theillumination means to the wave impedance of normal breast tissue; a waveguide for applying the input wave from the generator to the breastthrough the illumination means; focal point shifting means, connected tothe illumination means, for shifting the effective focal point acrossthe breast in a predetermined pattern to scan selected incrementalvolumes within the breast; at least one scattered wave collectorpositioned to intercept scattered waves from the incremental volumeswithin the breast and develop a return signal representative of thescattered waves from within the breast; and a receiver, including adetector, connected to the scattered wave collector, for detectinganomalies in the return signal to identify a tumor and its location inthe breast.
 2. A non-invasive system for detection of a tumor in abreast according to claim 1 and further comprising:separator means,connected to the scattered wave collector, for separating the scatteredwave signals from the input wave in developing the return signal.
 3. Anon-invasive system for detection of a tumor in a breast according toclaim 2 in which the separator means includes a circulator.
 4. Anon-invasive system for detection of a tumor in a breast according toclaim 2 in which the separator means includes polarization-responsivemeans for separating the input wave from the return signal based onpolarization of the return signal.
 5. A non-invasive system fordetection of a tumor in a breast according to claim 2 in which:theseparator means includes a pulse source of repetitive pulse signals; andthe system further comprises display means, connected to the pulsesource of the separator means and to the receiver, for displaying thereturns as a function of time; the rise time for each pulse signal beingno greater than 300 pico-seconds.
 6. A non-invasive system for detectionof a tumor in a breast according to claim 2 in which the pulse source isa part of a frequency-modulated pulse compression system that employs afrequency bandwidth of at least two gigahertz.
 7. A non-invasive systemfor detection of a tumor in a breast according to claim 2 and furthercomprising compensation means for compensating for change in thedielectric constant of the breast as a function of frequency.
 8. Anon-invasive system for detection of a tumor in a breast, according toclaim 2, in which a synthetic time-domain response is developed from theFourier inversion of the complex input impedance data as the frequencyof the illumination signal is progressively changed over a bandwidth ofat least two gigahertz.
 9. A non-invasive system for the detection of atumor, according to claim 8, and further comprising means to compensatefor the changes in the equipment behavior as a function of frequency.10. A non-invasive system for the detection of a tumor, according toclaim 8, and further comprising means to compensate for the path lossattenuation as the illuminating signal progresses into the breast.
 11. Anon-invasive system for detection of a tumor in a breast according toclaim 1, in which:the reflection receiver is a part of the illuminationmeans; and further comprising separator means, connected to thescattered wave collector, for separating the scattered wave signals fromthe input wave in developing the return signal.
 12. A non-invasivesystem for detection of a tumor in a breast according to claim 1 inwhich:the illumination means comprises a globular antenna filled with afirst dielectric medium; the illumination means further comprises aconnecting device, connecting the antenna to the breast, filled with asecond dielectric medium; and the dielectric constants of the breast,the first dielectric medium, and the second dielectric medium are allapproximately equal to each other.
 13. A non-invasive system fordetection of a tumor in a breast according to claim 12 in which theglobular antenna is of truncated ellipsoidal configuration and functionsas a part of the transmitter and a part of the collector.
 14. Anon-invasive system for detection of a tumor in a breast, according toclaim 12, in which the dielectric constants are in a range of four totwenty.
 15. A non-invasive system for detection of a tumor in a breastaccording to claim 1 in which the focal point shifting means includes amechanical scanner to change the location of the illuminator meansrelative to the breast.
 16. A non-invasive system for detection of atumor in a breast according to claim 15 and further comprising:focalpoint indication means, connected to the mechanical scanner; and imagingmeans, connected to the receiver and to the focal point indicationmeans, for displaying an image of the breast including a representationof any tumor therein.
 17. A non-invasive system for detection of a tumorin a breast according to claim 1 in which the connection from thegenerator to the receiver includes a phase shift circuit for shiftingthe input wave, as applied to one of the detectors, by an odd multipleof 90° relative to the input wave as supplied to the other detector. 18.A non-invasive system for detection of a tumor in a breast according toclaim 17 in which each of the detectors is a hybrid tee.
 19. Anon-invasive system for detection of a tumor in a breast according toclaim 1 in which the distance from the effective focal point to theeffective aperture of the illuminator is no greater than four times thediameter of the effective aperture.
 20. A non-invasive system fordetection of a tumor in a breast, according to claim 1, comprising atleast two scattered wave collectors, disposed on opposite sides of thebreast.
 21. A non-invasive system for detection of a tumor in a breast,according to claim 1, in which the receiver includes a directionalcoupler.
 22. An illumination means for a non-invasive system for thedetection of a tumor in a breast according to claim 1 comprising:anarray of a plurality of independently excitable wave guides, each havingan excitation end and an open end; means to note the position of thecontact point of each illuminator wave guide between the open end of thewave guide and the surface of the breast, with no more than a minimumspace between the wave guide and the breast; means to control the phaseangle of the input wave of each illuminator wave guide at its contactpoint in accordance with the position of the contact point; and means tochange the phase angle of the input wave of each illuminator wave guideat its contact point such that the individual waves that propagate fromeach of the illuminator wave guides into the breast add constructivelyat an effective focal point within a predetermined incremental volumewithin the breast.
 23. An illumination means for a non-invasive systemfor detection of a breast tumor according to claim 22 and furthercomprising means to move the array of wave guides and the interfacemechanically to scan the breast.
 24. An illumination means according toclaim 22 and further comprising:means to controllably and independentlyposition the open ends of the illuminator wave guides into predeterminedpositions on the surface of the breast.
 25. An illumination meansaccording to claim 22 in which the illuminator wave guides are eachfilled with materials that have dielectric characteristics similar tothat for the human breast.
 26. A scattered wave collector for anon-invasive system for the detection of a tumor in the breast,including an illumination means according to claim 22, that furtherincludes a scattered wave collector positioned to intercept wavesscattered from the effective focal point, comprising:an array of aplurality of collector wave guides, each having an open end; means tonote the position of the contact point of the open end of each collectorwave guide and the surface of the breast; and means to control the phaseangle of each of the waves collected by each collector wave guide suchthat the collected scattered waves that propagate from the effectivefocal point in the breast to each of the collector wave guides on thesurface of the breast are constructively combined.
 27. A wave collectionmeans for a non-invasive system for the detection of a tumor, accordingto claim 26, in which the illuminator wave guides are also the collectorwave guides.